Frequency hopping system for intermittent transmission

ABSTRACT

A radio transmission system including many radio transmitters using frequency hopping carriers to intermittently transmit very short messages indicative of status of stimuli associated with the transmitters. The transmitters transmit transmissions independently of a receiver receiving the transmissions and independent of each other. In operation, radio transmitters transmit messages at varying frequencies at time intervals that can be varied as well. The frequency and time intervals are varied according to patterns that can be determined individually for each transmitter. A receiver holds data indicative of the future transmission frequency and time for each transmitter and updates the data based on the time and the content of the received messages. In addition, a simple method is provided to generate a very large number of orthogonal frequency-time hopping sequences that are individual for each transmitter and based on the transmitter ID.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of, and is acontinuation-in-part of U.S. application Ser. No. 09/387,603 filed Aug.31, 1999 and entitled “Frequency Hopping System For IntermittentTransmission”, which is a continuation of U.S. Pat. No. 6,058,137, bothof which are incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to telemetry in general, and, moreparticularly, to a system in which a plurality of transmitterswirelessly transmit data for collection by one or more receivers.

BACKGROUND OF THE INVENTION

[0003] Some wireless telemetry systems (e.g., burglary alarms, firealarms, power utility meters, leak detectors, environmental monitoring,etc.) comprise many transmitters that periodically or sporadicallytransmit messages over radio frequencies to one or more receivers. Inthese systems, the transmitters are located at different places andtransmit messages that are indicative of the status of monitoringsensors to a receiver that collects the data from all of the sensors.Normally, the transmitters transmit messages that are as short asfeasible and with the interval between the transmissions as long asfeasible. This is advantageous for two reasons. First, it minimizes theaverage current drain in the transmitters, which are typically batteryoperated. Second, short and infrequent transmissions lower theprobability that the data is lost due to collisions that occur when twoor more transmitters transmit at the same time. However, if an alarmoccurs, the associated transmitter transmits immediately in order toconvey the alarm message with little delay.

[0004] Typically, such systems transmit data at a single frequency, andthus are susceptible to interference and signal loss due to a phenomenaknown as “multipath fading.” Consequently, the reliability of suchsystems is compromised or, conversely, the transmitted power has to beincreased to overcome the fading which results in larger power drain andshorter battery life. Besides, there usually are regulatory limits thatrestrict such transmitter power and thus limit the possible compensationby sheer increase of power. Since the multipath effect is highlysensitive to the frequency of the transmitted carrier, a system usingmultiple frequencies (e.g., a frequency hopping spread spectrum system,etc.) has a potential to eliminate these drawbacks. However, frequencyhopping systems require a long acquisition time and they are typicallyused in two way communication applications in which all the devices arecontinuously synchronizing with one master device or with each otherusing a variety of synchronization methods as shown in some references.In other cases, to ease the synchronization problem, there are employedreceivers that can simultaneously receive signals at many frequencies bymaking the receiver broadband or by using several receivers at the sametime. Generally, those Solutions suffer from performance degradation orhigh cost or both which makes them undesirable for low cost applicationsthat require high reliability such as security systems and sometelemetry systems.

[0005] A serious problem that must be addressed in battery operatedsystems is to shorten the transmission time as much as possible bymaking the message preamble as short as possible in order to conservethe battery power. Therefore, the synchronization of the receiver withthe transmitters is a difficult task. This problem is exacerbated insome systems such as security alarms that require some messages to beconveyed to the system immediately without waiting for the scheduledtransmission time.

[0006] A related problem, in battery operated systems, is limitation ofthe transmitted power to conserve the battery power.

SUMMARY OF THE INVENTION

[0007] Some embodiments of the present invention comprise a frequencyhopping receiver that acquires and maintains synchronization with aplurality of transmitters, which enables the transmitters to omit thetransmission of long preambles. This is advantageous because it lowersthe average current drain in the transmitters and, consequently,lengthens their battery life. Furthermore, some embodiments of thepresent invention are advantageous in that they provide improvedreliability in the presence of multipath fading, interference andjamming. And still furthermore, some embodiments of the presentinvention are capable of eliminating the effect of persistent collisionsthat occur when two or more transmitters transmit at the same time inthe same channel for a prolonged period.

[0008] The illustrative embodiment of the present invention is afrequency-hopping wireless telemetry system comprising: (1) one or morereceivers, and (2) one or more transmitters, each of which receive inputfrom one or more sensors. The transmitters intermittently transmit overradio frequencies very short messages indicative of status of thesensors associated with the transmitters. Each transmitter includes atime interval generator to establish the time interval betweensuccessive transmissions, a frequency synthesizer-modulator to generatea modulated radio frequency carrier signal wherein the frequency of thecarrier changes in response to programming the synthesizer by digitaldata, a reference frequency oscillator to provide a frequency referencefrom which the synthesizer derives carrier frequencies and,advantageously, from which the time interval generator derives itstiming, and a transmitter control logic activated in response to pulsesfrom the time interval generator or a sensor signal indicating anabnormal condition. When activated, the transmitter control logicactivates and programs the synthesizer so that the transmitter carrierfrequency is changed according to a frequency hopping algorithm,provides digital data indicative of the sensor status and advantageouslybattery status, and modulates the carrier with the provided data. Thereceiver includes a frequency selective radio receiver circuit,programmable by digital data, to receive and demodulate a transmittedcarrier when the frequency of the receiver circuit is programmedaccording to the frequency of the carrier, and a receiver control logicmeans to process demodulated data, to provide system interfaceresponsive to the received messages, and to program the frequency of thefrequency selective receiver circuit. The control logic includes areceiver timer to measure the elapsing time, and a plurality of memoryregisters to hold digital data indicative of (a) the time of the nexttransmission occurrence for each transmitter and (b) the frequency ofthe next transmission occurrence for each transmitter. In operation, thecontrol logic sequentially compares the data content of the timeregisters with the data content of the timer and if the transmission isdue from a transmitter, the control logic programs the frequencyselective radio receiver circuit according to the data content in thefrequency register associated with said transmitter, attempts to decodethe demodulated signal, modifies the content of the time register by anumber representative of the time interval between the successivetransmissions for said transmitter and modifies the content of thefrequency register according to a predetermined algorithm for saidtransmitter.

[0009] In accordance with the illustrative embodiment of the presentinvention there is provided a method of transmission in the system so asto improve reliability of the system in the presence of multipath fadingand interference, the method is based on varying the transmissionfrequency for each transmitted message and varying the time betweenconsecutive messages. The frequency variations provide frequencydiversity and are effective against multipath fading as well as singleof multiple narrowband interference. The time variations are effectiveagainst periodic impulse interference. In combination, the frequency andtime variations provide immunity for a wide variety of signalimpairments and interference including multipath fading, wide andnarrowband interference, impulse noise and deliberate jamming.

[0010] In accordance with the illustrative embodiment of the presentinvention there is provided a method of minimizing the effect ofcollisions, the method is based on selecting the transmissionfrequencies in sequences that are different for each transmitter,wherein transmitter frequency sequence depends oil the transmitter IDnumber or other number derived or associated with the transmitter ID. Inaddition, in the illustrative embodiment, the transmitter ID number orother number derived or associated with the transmitter number isincluded in the transmitted message, so that, upon reception of a singlemessage from a transmitter, the receiver can determine what is the nextfrequency for this transmitter, and thus achieve synchronization withthis transmitter.

[0011] In accordance with the illustrative embodiment of the presentinvention there is provided another method of minimizing the effect ofcollisions that can be used alone or in conjunction with the thirdaspect of this invention, the method comprising randomizing the timeinterval between transmissions individually for each transmitter and areceiver compensating for the time interval changes.

[0012] In accordance with the illustrative embodiment of the presentinvention there is provided a simple method to generate a very largenumber of frequency-time hopping sequences. The method producessequences that are orthogonal, thus eliminates possibility of persistentcollisions even when large number of transmitters are used. In addition,the method requires identical circuit in each transmitter and the actualsequence that is produced is selected by the transmitter ID or othernumber associated with the transmitter ID, thus making it convenient formanufacturing. Also, the method enables to produce a very large numberof frequency-time sequences based on a single short PN generator whosestate can be instantly recovered by a receiver based on just onereceived transmission, thus aiding the receiver in obtainingsynchronization with a transmitter whose ID is known. At the same time,because of a very large number of possible sequences that can begenerated, it is difficult to obtain synchronization if the transmitterID is not known, which makes the system immune to interception andjamming.

[0013] In accordance with the illustrative embodiment of the presentinvention there is provided a method that enables such a system toconvey the information about an abnormal sensor condition as soon as thecondition occurs regardless of the transmission period of the associatedtransmitter. The method is based on selecting an alarm frequency oradvantageously a group of alarm frequencies common for all transmitters.The alarm frequencies are used by the transmitters when an alarm or anabnormal sensor condition occurs or on power-up, wherein when such acondition occurs in a transmitter, the transmitter transmits themessages sequentially on the alarm frequencies for a predeterminedperiod of time after which the transmitter resumes transmissionsaccording to the sequence before the alarm condition, wherein thereceiver monitors the alarm frequencies during the time between thereception of scheduled messages from the transmitters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of a transmitter according to aadvantageous embodiment of the present invention.

[0015]FIG. 2 is a block diagram of a receiver according to aadvantageous embodiment of the present invention.

[0016]FIG. 3 is a block diagram of an example of illustrative embodimentof a sequence generator used to determine the individual frequencysequences.

[0017]FIG. 4 is a block diagram of an example of a illustrativeembodiment of a sequence generator used to determine the individualfrequency-time sequences.

[0018]FIG. 5 is a block diagram depicting the frequency hopping systemincluding many transmitters and a receiver.

DETAILED DESCRIPTION

[0019] Referring to FIG. 5, the frequency hopping system includes aradio receiver 401 and a plurality of radio transmitters 402, 403, 404and 405. The radio receiver includes a system interface 410 throughwhich the receiver can be connected to a variety of interface equipment,a controller, or a computer. Each transmitter includes an interface or asensor or an operation to be monitored. Each transmitter intermittentlytransmits short messages to the receiver respectively. The transmittersare not connected to each other and do not receive messages back fromthe receiver. The transmitters transmit messages when they need towithout any regard to other transmitters, as the transmitters are notsynchronized with each other.

[0020] Referring to FIG. 1, the transmitter includes a referencefrequency crystal oscillator 6 to produce a stable frequency on line 26,a time interval generator 2 establishing a time base to produce pulseson line 28 activating the transmitter, a frequency synthesizer-modulator4 to produce a radio frequency carrier modulated by modulation data fedto the synthesizer via line 24 wherein the frequency of the carrier isprogrammed to a desired value via plurality of lines 14, transmittercontrol logic 8 to activate and program the synthesizer-modulator 4 viaplurality of lines 14 when the logic is activated by a pulse from thetime interval generator or by an abnormal signal indication on a sensorsignal input line 18, an amplifier 10 to amplify the radio carrierprovided by the synthesizer when the amplifier is activated by thecontrol logic 8 via line 16, and an antenna 12 to radiate the powerdelivered by the amplifier. The control logic 8 includes a frequency andtime data memory register 20 to hold information used to determine thetime and the frequency of next transmission, and a sensor interfacecircuit 22 to accept the sensor signal and detect an abnormal signalcondition, and to convert the sensor signal to a digital format suitablefor transmission. The transmitter logic also includes a storage means 30to store a transmitter ID number to differentiate this transmitter fromother transmitters. The transmitter control logic, in some systems, canbe realized based on a microprocessor, in some other systems, aspecialized application specific integrated circuit—ASIC component maybe used.

[0021] In operation, during the time between transmissions, thetransmitter is in a standby mode in which the amplifier 10 andsynthesizer-modulator 4 are not active and, advantageously, the controlsignals turn off the power from these circuits in order to minimize thestandby current of the transmitter. The transmitter control logic 8 isin a standby mode in which the most of the circuits are inactive andsome or most of the circuitry can be powered down with the exception ofthe circuits supporting critical functions; (a) the sensor interfacecircuit 22 that detects an abnormal signal condition and produces abinary signal that is logically combined with the signal 28 produced bythe time interval generator so that when either a pulse or abnormalcondition occurs the rest of the transmit logic circuitry is activatedor powered up, (b) the frequency and time data memory 20 that has toretain the data during the period between transmission and consequentlyeither it has to be a nonvolatile type or it has to be powered up duringthe period between transmissions. Upon activation, the control logic 8determines the activation source by reading signals 28 and 18.

[0022] When the logic 8 is activated by a pulse 28 from the timeinterval generator the following sequence of events occurs. First, thelogic reads the frequency data memory and produces a data packet thatincludes the sensor status, the transmitter ID number and other datasuch as battery status. Then, the logic activates and programs thesynthesizer-modulator 4, activates the amplifier 10 and sends the packetto the modulator via line 24. After completion of each transmission, thetransmitter logic sets the transmitter in the standby mode untilactivated again by a pulse on line 28 or a sensor abnormal conditionindicated on line 18.

[0023] In the advantageous embodiment the transmission of a packet canbe repeated a predetermined number of times at separate frequencies,wherein the number of repetitions is chosen according to applicationneeds and, wherein the frequencies are determined by the transmitterlogic according to an algorithm described later in details. This way, itis possible for the receiver to receive some repeated packets even ifthe other packets are lost due to frequency selective fading caused bymultipath or due to interference. Similarly, a single packet can besplit into several pieces and each piece transmitted at a separatefrequency. In yet another embodiment, it may be advantageous to use morethan one carrier at the same time to improve reliability, however thiswould require a more complex transmitter.

[0024] When a sensor abnormal condition occurs, the sensor interfacecircuit 22 produces an active level of the signal indicative of thesensor abnormal level which activates the transmitter via acombinatorial logic circuit that combines the sensor abnormal levelsignal with the pulses from the time interval generator. When activatedthis way, the transmitter control logic 8 produces a data packet thatincludes the sensor status, then the logic activates and programs thesynthesizer-modulator 4, activates the amplifier 10, and sends thepacket to the synthesizer-modulator. In the advantageous embodiment, thetransmission of the alarm packet is repeated a predetermined number oftimes using a plurality of predetermined alarm frequencies in such a waythat the transmission frequency is changed after each single packettransmission according to a predetermined fixed sequence. In theillustrative embodiment, when the alarm packets are transmitted, thetime intervals between transmissions are minimal; when one transmissionis completed, the transmitter programs to the next frequency after asmall predetermined interval and repeats the packet transmission, etc.It should be noted that the existence of the predetermined alarmfrequencies is not necessary albeit advantageous. In an alternativedesign, the transmitter may follow the normal hopping pattern but at anincreased rate repeating the alarm message a predetermined number oftimes. The essence of the idea is that the alarm message beinginfrequent can afford a much greater transmission overhead and can berepeated many times. If the alarm message is transmitted at fewerfrequencies, a faster response of the receiver will be observed onaverage.

[0025] After the transmission sequence is completed, the control logicdisables the signal indicative of the sensor abnormal status so that anabnormal sensor status can not activate the control logic. Then, thecontrol logic puts the transmitter in the standby mode until activatedby a pulse from the time interval generator. When subsequentlyactivated, the transmitter control logic performs the usual transmissionsequence but the data packets include information that the sensorcondition is abnormal if the condition persists. When the abnormalcondition subsides, the signal indicative of an abnormal status isenabled so that a subsequent occurrence of an abnormal condition canactivate the logic and trigger a new alarm transmission sequence. Thus,normal operation is restored.

[0026] Although FIG. 1 shows a specific illustrative embodiment, it isapparent that various modifications may be realized such as includingmore than one sensor, placing different ID in each sensor, or evenplacing the entire control logic in a sensor, or combining transmitterand sensor, etc.

[0027] In the advantageous embodiment, the sequence in which thefrequencies are used is different for different transmitters. Thefollowing is the description how this is accomplished in theadvantageous embodiment. Each transmitter includes a pseudo randomsequence generator or pseudo noise—PN generator, wherein a pseudo randomsequence generator is based on a linear feedback shift register, andwherein some outputs of the shift register are fed back to an EX-OR(Exclusive OR) gate whose output is connected to the register input. Fora certain combination of the outputs that are fed to the EX-OR gate, theshift register can produce a sequence that has 2^(N)-bits, wherein N isthe length of the shift register. Such a sequence is called a maximumlength sequence. Alternatively, if all the outputs of the shift registerare taken at a time, then a pseudo random sequence of 2^(N)−1 numbers iscreated, wherein all the numbers have N digits and each number differsfrom all the other numbers in the sequence; the numbers range from 1 to2^(N)−1. For example, a generator based on a 3-bit shift registerproduces a sequence consisting of 7 3-bit numbers. The numbers rangefrom 1 to 7. Such PN generators are well known to the skilled in theart.

[0028] Referring to FIG. 3, the pseudo random sequence generator 203consists of a shift register 205 and EX-OR gate 204. The shift register205 is composed of three stages 221, 222, and 223 having three outputsQ₀ 211, Q₁ 212 and Q₂ 213 respectively. The feedback is taken fromoutputs Q₀ and Q₂. The three least significant bits of the transmitterID {i₂, i₁, i₀} 201 are combined with the output of the pseudo randomsequence generator {Q₂, Q₁, Q₀} using EX-OR gates 208, 207, and 206. Theresult can be used to indicate the frequency or frequency channel orfrequency index {f₂, f₁, f₀} 202 indicative of the frequency over whichthe transmission will occur.

[0029] Assuming that the initial state of the shift register is binary111 (decimal 7; Q₂=1, Q₁=1, Q₀=1), the produced sequence is {7, 3, 5, 2,1, 4, 6}. These numbers are then combined with the last three bits ofthe transmitter ID using bit by bit EX-OR operation; i.e. the last bitof the transmitter ID (i₀) is combined with the last bit of the randomnumber (Q₀), etc. This way produced new sequence has numbers rangingfrom 0 to 7 the order of which depends on the last three bits of thetransmitter ID. Thus, 8 distinct (permuted) sequences of numbers arecreated.

[0030] For example, if the last digits of the transmitter ID are 000,then the frequencies are selected in the order 7, 3, 5, 2, 1, 4, 6, i.e.the sequence is not altered. If the last three digits of the transmitterID are 001, then the frequencies are selected in the order 6, 2, 4, 3,0, 5, 7; if the last three digits of the transmitter ID are 010, thenthe frequencies are selected in the order 5, 1, 7, 0, 3, 6, 4; etc.Notice, that each newly created sequence is not, strictly speaking, apermutation of the basic sequence because in each new sequence onenumber is converted to 0; e.g. in the first example 1 was converted to 0in the process, in the second example 2 was converted to zero. However,for the purpose of this application the operation is regarded and calleda “permuting operation” or a “permutation”, or “permutation process”,etc. Similarly, the resulting sequence is called “permuted sequence”.

[0031] In the illustrative embodiment, the sequence length depends onthe number of frequencies used by the system. For example, if theavailable bandwidth is 26 MHz and the frequencies are separated by a 100kHz interval, then there are 260 frequencies—channels available fortransmission. I.e. the sequence length can be 255 by using an 8-bitshift register. By processing the output of the PN generator with 8transmitter ID bits, a 255 different frequency sequences are obtained.

[0032] It is apparent that any two sequences are quite different eventhough the ID number is changed only on one position. In fact, thesequences are orthogonal, i.e., for any two sequences, a coincidence oftwo numbers occurs only once for the entire period of the PN generator.

[0033] This is advantageous since it lowers the probability ofpersistent collisions that may happen if two or more transmitterstransmit at the same time and at the same frequency for a prolongedtime. It should be stressed that using the sequences as describedensures that the persistent collision between any two transmitters isnot possible since the frequencies in any arbitrary pair of sequences donot coincide persistently regardless of the relative shift of thesequences.

[0034] For each transmitter, the future frequency can be predicted basedon just one partially received message because each message includes thetransmitter ID based on which the receiver can determine the content ofthe frequency index generator. To see how this can be done one needs tonotice that the permutation process according to the illustrativeembodiment is reversible: i.e., if the same ID digits are applied againto the frequency index, the PN generator state is obtained. If thereceiver receives a least a part of a message from a transmitter, thefrequency index is known. It is only necessary to know the digits of thetransmitter ID. The transmitter ID digits are included in the messages.Advantageously, they are placed at the end of the message so that thereceiver can recover them even if it starts the reception of thetransmission in the middle of the message.

[0035] In some applications, this number of sequences my not besufficient. For example, in many applications, the transmitter IDs aregenerated sequentially in the factory and embedded in the transmittercircuit. It would not be convenient or sometimes not even possible tomake sure that all the transmitters to be installed on one premise or ingeographical proximity would produce orthogonal sequences. In such casesthe number of sequences can be extended to a larger number using othertechniques, some of which were extensively studied and are described inthe available literature. The number of available permutations of asequence that has 127 numbers is 127! (≈3E213). Even if a small subsetof all the available permutations is used, the will be a large andadequate number of frequencies produced. This way, the manufacturer canstill embed sequentially produced ID numbers and the transmitters couldstill be used without regard to the sequences that they produce.However, the method described above is advantageous for its simplicityand the unique properties or orthogonality of all sequences. The degreeof orthogonality indicates how many hits (frequency agreements) theremay be between two sequences upon any relative cyclic shift of thesequences. In a perfect design, for any two sequence that use the sameset of frequencies, there would be only one hit. I.e., if upon anycyclic shift of two sequences, a position is found in which the samefrequency is present in both sequences, then the frequencies in allother positions would differ. The sequences produced in a manner asdescribed in the advantageous embodiment are orthogonal in that sense.Although perfect orthogonality is not necessary for proper operation ofthe system, it is desirable since it reduces the probability of lostpackets due to collisions. However, it should be apparent that otherways of permuting the sequences could be created. Another advantage ofthe advantageous method of permutation is that the permutation processis reversible as previously described. This property enables thereceiver to synchronize based on the transmitter ID embedded in thetransmitted messages without a need for additional information bitsrelated to the PN generator status. Since the transmitter ID is normallyrequired in the transmitted messages, the advantageous method does notrequire any overhead in the transmitted messages.

[0036] In order to preserve the property of orthogonality and zerooverhead as described above and enlarge the number of transmitter IDuniverse the following advantageous method is employed.

[0037] Normally, the time intervals between transmissions are controlledby a quartz crystal and, ideally their nominal values are the same forall transmitters, however in the advantageous embodiment, the timeintervals are perturbed by small time increments to further randomizethe transmission events and lower the probability of persistentcollisions with other transmitters as well as avoiding an intentional orunintentional pulsed interference. The transmitter control logic canaccomplish this by programming the time interval generator via line 27(FIG. 1) according to a predetermined algorithm. The information aboutthe current status of the algorithm may be included in the transmittedpacket to aid the receiver operation.

[0038] In the advantageous embodiment, the method of determining thetime interval perturbation is based on a similar technique as describedin conjunction with the frequency index generation, wherein the randomsequence is used to alter the time interval between transmissions. I.e.,each time a transmission is performed, a new number is generated andused to determine the time interval between the current and the nexttransmission. Wherein, the time randomization is accomplished byprocessing the output of the PN generator used for the frequency indexwith bits of the transmitter ID. The processing is done as follows.

[0039] Referring to FIG. 4, the frequency index is produced by the PNgenerator 203 outputs 213, 212 and 211 and transmitter ID bits 201—{i₂,i₁, i₀} processed with EXOR gates 208, 207, and 206 to produce indexdigits 202—{f₂, f₁, f₀} as described previously in conjunction with FIG.3. The PN generator output is further processed with transmitter IDdigits 302—{i₅, i₄, i₃} by the AND gates 308, 306 and 304 and by an EXORgate 310. The output of the gate 310 taken one bit at the time is ashifted replica of the output of the PN generator e.g. output 211 or 212or 213. Where the relative shift depends on the transmitter ID digits302. The output of the gate 310 is then fed to a shift register 312whose outputs 323, 322 and 321 are shifted replicas of the PN generatoroutputs 213, 212, and 211 respectively. When taken three digits at thetime, the sequence produced at the output of the shift register is ashifted replica of the output of the PN generator. For example, if thePN sequence produced is order {7, 3, 5, 2, 1, 4, 6}, and bits i₅, i₄, i₃are 011 then the shifted sequence is {4, 6, 7, 3, 5, 2, 1}; if the bitsi₅, i₄, i₃ are 101 then the shifted sequence is {2, 1, 4, 6, 7, 3, 5}.This way, a total of 7 shifted sequences are produced (000 input is notallowed). The shifted sequences are further processed with bits i₈, i₇and i₆ 320 of the transmitter ID by EXOR gates 318, 316, and 314 toproduce permutations of the shifted sequences at the outputs 333—{t₂,t₁, t₀} in a manner identical to the previously described in conjunctionwith frequency index generation. This way, each shifted sequence can bepermuted in 8 different ways creating total 7*8=56 shifted-permutedsequences. The shifted and permuted sequences are used to producevariations of the time between consecutive transmissions. In theillustrative embodiment, the numbers from a sequence are multiplied by adT=2*Tm and added to the nominal time between transmission—TBT. Where,Tm is the nominal message transmission time. Advantageously, dT isrounded to the nearest discrete multiple of a basic time measure unitused by the control logic. If the permuted PN sequences are used asfrequency indexes and the shifted-permuted sequences are used torandomize the time between transmission, then there are created8*7*8=448 sequences that are time-frequency orthogonal in the sense thatif two sequences coincide at one frequency and time, they will notcoincide for any other frequency and time for the entire PN generatorperiod. This is based on merely 3-bit generator of the illustrativeexample! Of course, if a longer shift register is used for the PNgenerator, a far greater number of sequences are created. In theillustrative embodiment, an 8-bit generator is used as describedpreviously. This results in over 16E6 orthogonal time-frequencysequences. Enough to relieve the manufacturer and applications fromtransmitter ID management other than sequential numbering of allmanufactured transmitters. Of course, for a 8 bits shift register, 8bits of the transmitter ID are used to obtain sequence permutation forfrequency index, similarly, 8 bits are used for shifting and another 8bits for permuting the shifted sequence to obtain the time delayvariations.

[0040] The essence of this method is that in addition to two apparentdimensions of variability present in the form of permutations offrequency and time sequences, there is a third dimension added: i.e. thephase relationship variability between the frequency and time sequences.This rapidly increases a number of distinct orthogonal frequency-timesequences with increasing length of the basic PN generator as evidencedby the illustrative example. While it is possible to use other kinds ofbasic sequence and other ways of transforming the numbers of the basicsequence to obtain new sequences, the added new dimension has severaladvantages as evidenced in the illustrative embodiment.

[0041] The permutation process as described is an example of a moregeneral process of transformation that transforms a set of numbers intoanother set of numbers (that may differ in size). It should be apparentthat although a transformation resulting in the permutation as describedis advantageous, other transformations may be used to derivefrequency-time pattern based on the described principle.

[0042] It should also be apparent that in some implementations the orderin which the shift and the second transformation (permutation) isperformed may be reversed without altering the essence of the method.

[0043] Another advantage of the illustrative embodiment is that thepermutations and shifting of the sequences can be performed byprocessing (transforming) one number of the sequence at the time, thuseliminating the need to store and manipulate the entire sequence. I.e.,the permuted or shifted sequence numbers are produced one at the time asneeded based on numbers from the basic sequence that are also producedone at the time as needed.

[0044] Note, that this advantageous way of producing the frequenciesdoes not require any overhead in the transmitted messages for thesynchronization purpose other than the transmitter ID that is normallyrequired any way. This is because the receiver can instantly recover thePN generator status based on just a single received message. Asdescribed previously, the receiver can infer the status of the 8-bitgenerator based on the received frequency index and the transmitter IDnumber. I.e. the message contains the information about the 8-bitgenerator without explicit inclusion of the generator status bits in themessage. In the illustrative embodiment, after the frequency index isobtained for a transmission, the time index is obtained by filling theshift register in the steps of storing the PN generator status, clockingthe PN generator and shift register N times, and restoring PN generatorstatus. This way, the content of the shift register 312 is not requiredby the receiver to obtain synchronization because the time index dependson the future content of the PN generator that can be easily duplicatedin the receiver based on the present content. Therefore, the receivercan still synchronize with a transmitter based on one received messageand the message does not need to include any overhead forsynchronization.

[0045] In an alternative implementation, a second PN generatorsynchronized with the first PN generator may be used to produce the timevariations wherein an information about the second generator phase isincluded in the transmitted message to aid the synchronization. Note,that synchronization of the first and the second generator in thetransmitter is extremely important since the essence of the idea is thatthe cyclic shift of the second sequence is provided in respect to thereference provided by the phase of the first sequence. Only this way theresulting frequency-time hopping sequences produced in differenttransmitters are orthogonal.

[0046] Although, the described implementation based on a singlegenerator is advantageous since it results in a simpler implementationand lower overhead leading to a longer battery life, the two generatorimplementation can be modified to ensure low overhead as follows.

[0047] Both, first and second PN generators produce basic sequenceswhose length is 2^(N)−1 and 2^(M)−1, wherein N and M are the lengths ofthe respective shift registers in both generators. In order to providefor synchronization between both sequences, each sequence is extended byone bit by inserting one “0” bit at a predetermined place in thesequence. The advantageous place is after N−1 or M−1 “0” bits in therespective sequences. This way the lengths become 2^(N) and 2^(M)respectively which ensures that both sequence lengths are related by apower of 2 (i.e. 2, 4, 8, etc.). Now, it is possible to ensure that bothsequences are always in the same phase relation; e.g. after initialreset which sets the generators in a predetermined state, bothgenerators are advanced at the same time. This way, they will return tothe exact initial state after the full period of the longer sequence. Ofthe particular interest is the case of the time generator producing alonger sequence than the frequency generator. In some applications,there is a limited number of frequency channels available, however thereis still a need to produce a large number of frequency-time orthogonalsequences. In such case, a longer time sequence can be used to expandthe number of possible frequency-time sequences. For example if thefrequency generator shift register has N bits and the time generatorshift register has M bits, then the total number of sequences is2^(N)*(2^(M)−1)*2^(M) as shown in the preceding examples. Each time M isincreased by one, the number of frequency-time sequences is enlargedapproximately by a factor of four resulting in a rapid increase of thenumber of sequences with the increase of the time generator shiftregister length. Also, the synchronization requires a small overheadbecause the receiver can infer the frequency generator state and needsonly the state of the time generator. However, if the time generator isin precise phase lock with the frequency generator, the transmitter doesnot need to send the actual time generator state. Instead, thetransmitter needs to include the information to remove the uncertaintycreated by the time sequence period being multiple of the frequencysequence period. E.g., if the time sequence is two times longer, thereceiver needs to know if the time generator is in the first half or thesecond half of the sequence to determine the exact state of the timegenerator. In this particular case, this information requires only onebit to be included in the transmitted messages. Of course, more bits arerequired if the time sequence is longer, e.g. if the time sequence is 4times longer than the frequency sequence, two bits are required; for 8times longer sequence 3 bits are needed, etc.

[0048] The described method (with one or two generators) produce a largenumber of time-frequency orthogonal sequences in a simple and systematicway that enables the sequence selection by the transmitter ID andrequires zero (or very small) overhead for synchronization. A systemusing a large number of time-frequency orthogonal sequences as describedhas an advantage of immunity to multipath fading, pulsed and frequencyselective interference including intentional jamming, as well as lowprobability of self interference due to persistent collisions that mayoccur when two or more transmitters transmit messages on the samefrequency and at the same time for a prolonged period. A large number ofproduced frequencies enables the manufacturer and the system operatorsnot to be concerned with the management of sequences for all thetransmitters. Instead, each manufactured transmitter can produce aunique sequence that can be easily replicated in the receiver based juston the transmitter ID.

[0049] It is to be understood that the random frequency selection asdescribed above and the time perturbation can be used together or inseparation to achieve immunity to collisions. I.e. (a) a fixed frequencypattern for all transmitters and random time perturbation patternsindividual for each transmitter can be used, or (b) a fixed timeinterval between transmission or fixed time perturbation pattern andrandom frequency selection individual for each transmitter can be used,or (c) frequency and time changes can be combined to enhance the systemperformance at the expense of complication.

[0050] In the advantageous embodiment, both the transmission frequencyand the time interval between transmissions are individually randomizedfor each transmitter by the transmitter ID bits.

[0051] It is also to be understood that one does not have to use thetransmitter ID bits to individually predetermine the frequency and/ortime patterns for each transmitter. In an alternative design, a randomseed can be generated in the transmitter, for example just after reset,and used in lieu of the ID number to modify the frequency and timepatterns. If the random seed has many bits, the probability ofgenerating the same pattern by two transmitters in the system is verysmall. However, this solution is considered inferior because it requiresthat additional steps are taken to associate the random seed number withthe transmitter ID. In addition, this solution requires a good truerandom number generator that produces numbers with roughly the sameprobability in order to prevent frequent repetitions of some numbers.Also, although the probability of sequence repetition is small, it maybe necessary to include an additional step in the process installing thetransmitter to reject a seed number that is already used by anothertransmitter. All this increases the complexity and the cost andpotentially makes the installation more difficult.

[0052] It is also to be understood that the illustrated method and itscomponents such as generators, registers, gates, etc., can be realizedin various forms of hardware some of which may include ASIC, orsoftware, or their combination.

[0053] Referring to FIG. 2, the receiver includes a reference frequencycrystal oscillator 126 to produce a stable reference frequency on line128 for the receiver circuits, a frequency selective radio receivercircuit 100 whose frequency is programmable via lines 116, to receiveand demodulate a frequency modulated carrier when the frequency of thefrequency selective receiver circuit is programmed according to thefrequency of the carrier, and a receiver control logic means 130 toprocess demodulated data, to provide system interface lines 140,responsive to the received data, and to program the frequency of thefrequency selective receiver circuit. The control logic includes areceiver timer 132 establishing a time base to measure the elapsingtime. The control logic also includes: (a) a plurality of ID memoryregisters 134 to hold digital data indicative of ID numbers for eachtransmitter that belongs to the system, (b) a plurality of time memoryregisters 136 to hold digital data indicative of the time of the nexttransmission occurrence for each respective transmitter, and (c) aplurality of frequency memory registers 138 to hold digital dataindicative of the frequency of the next transmission occurrence for eachrespective transmitter. In the advantageous embodiment, the registersare organized such that an arbitrary register i 151 of the plurality ofID memory registers 134 associated with a transmitter whose ID number isn, is associated with register i 152 of the plurality of time memoryregisters 136 and register i 153 of plurality of frequency memoryregisters 138, wherein said registers 152 and 153 hold data associatedwith said transmitter n. The frequency selective radio receiver circuit100 includes a RF band pass filter 104, an amplifier 106, an IF bandpassfilter 110, a mixer 108, limiter-discriminator circuit 112 and frequencysynthesizer 114. The RF band-pass filter selects only the desiredfrequency band allocated for the transmission, the mixer mixes theincoming signal with the signal produced in the frequency synthesizerand produces an IF frequency (Intermediate Frequency). The IF frequencyis filtered in a narrow band filter 110 whose bandwidth is selectedaccording to the channel bandwidth. The limiter discriminatordemodulates the signal and produces baseband DATA signal 120 and an RSSIsignal 118 indicative of the received signal strength. The DATA signal120 and the RSSI signal 118 are converted to binary signals by A/Dconverters 124 and 122 respectively and fed to the control logic 130.The presented architecture of the frequency selective radio receivercircuit 100 is known as a superheterodyne FM receiver, it is very wellknown and it does not require additional explanation. The transmittedmessage data is extracted from the DATA signal 120 digitized by the A/Dconverter 124 using one of the many well-known methods for signalprocessing and does not require additional explanation.

[0054] In the advantageous embodiment, the frequency registers 138 holdfor each transmitter the state of the PN generator used by thetransmitter to produce the frequency indexes and time variations. If thesynchronization is obtained with a given transmitter, the state of thePN generator is identical to that in the transmitter. In theillustrative embodiment, the time registers 136 hold numbers—time ofnext transmission—for each transmitter representing the state of thereceiver timer 132 at the time the next transmission is due from atransmitter.

[0055] In operation, the receiver control logic 130 sequentiallycompares the data content of the time registers 136 with the datacontent of the receiver timer 132 and if the transmission is due from atransmitter, the control logic programs the frequency selective radioreceiver circuit 100 according to the data content in the frequencyregister 138 for this transmitter, attempts to decode the demodulatedsignal, changes the content of the time register based on the numberrepresentative of the time interval between the transmissions for thistransmitter and changes the content of the frequency register accordingto a predetermined algorithm for this transmitter. I.e. the frequencyand the time registers are updated each time a transmission is dueregardless whether the packet was received successfully. The new contentof the frequency register is determined according to the algorithm forthe frequency use by the transmitters.

[0056] The new content of the time register is calculated based on thecurrent content of the receiver timer and a number representative of thetime between the current transmission and the next transmission for thistransmitter, wherein said number is calculated based on the nominalvalue of the time between the transmissions and adjusted by the pseudorandom perturbation performed according to the previously describedalgorithm. In addition, said number is corrected by a correction factorbased on the measured difference between the transmitter time base andthe time base of the receiver, wherein said difference is determined ina manner described later in details. In the advantageous embodiment, thenumbers representative of the time base differences are stored in thetime registers 136 separately for each transmitter and are independentfrom the numbers representing the time of the next transmission, i.e.the time registers are split to hold two independent numbers.

[0057] It should be noted that even if crystal oscillators are used inthe transmitters and the receiver to control the timing, the erroraccumulated during the time between transmissions can be significantcompared to the packet time. For example, if the nominal period betweenthe transmissions is 100 seconds and the crystal frequency error due totolerance and temperature changes is +/−20 ppm (parts per million) forthe transmitter and +/−10 ppm for the receiver, then the error may be aslarge as 3 ms. If the time for the transmission of one packet is 5 ms,then the error is significant. In reality, such tight tolerance isdifficult to achieve and expensive and therefore in many applications amuch bigger error will be accumulated. In order to minimize the timeerror accumulated during the long time between the transmissions, thereceiver can store the time difference between the ideal and the actualtime of the packet reception and use the difference to predict moreaccurately the next transmission time. For example, if the timerresolution is 0.25 ms, then the next transmission time can be predictedwith accuracy 0.25 ms, providing that the temperature does not changeappreciably over 100 s period. This represents an improvement of anorder of magnitude. I.e. the receiver can program its frequency 0.25 msin advance to each new frequency, examine it for the duration of thepacket, then program to the next frequency and so on.

[0058] During the acquisition, when the time error is not known, thereceiver needs to tune to the first frequency at least 3 ms in advance.Then, the receiver monitors the received signal by observing the RSSIsignal 118 and DATA signal 120. If during the next 6 ms no valid signalis present, the receiver programs to the next frequency 3 ms in advanceand so on. Of course, if worse tolerances are used, the time thereceiver dwells on a single frequency expecting the next transmission isproportionately longer.

[0059] Some embodiments of the present invention relates to the readingof electric utility meters. In such embodiments, the transmittersadvantageously use the power-line as a frequency reference from whichthe time intervals between transmissions are derived. The advantage ofthis arrangement is that all transmitters time drifts in the sameproportions relative to the receiver. This can ease the receiver task ofacquisition as well as tracking.

[0060] However, in the illustrative embodiment, to ease this acquisitionproblem, the receiver includes a frequency error detection means and amethod as described below.

[0061] In the advantageous embodiment, the receiver includes a frequencyerror detection means 142 that is advantageously implemented as a simpledigital counter, in order to detect the frequency error in the receivedsignal in respect to the receiver reference frequency by measuring thefrequency error of the intermediate frequency signal 111. In addition,in the transmitter, the transmitted carrier frequency and the timeinterval generator timing are derived from the same source and in thereceiver, the receiver frequency and the receiver timer are derived fromthe same reference. In operation, the receiver can measure the frequencydifference between the transmitted carrier and the receiver frequencyand use the measured error to determine the difference between thetransmitter reference frequency and the receiver reference frequencybased on just one partially decoded message. The frequency differencemeasurement is accomplished in the following way. Assuming that thetransmitter frequency accuracy is +/−20 ppm and the receiver is +/−10ppm, the carrier frequency is 915 MHz and the IF frequency is 10.7 MHz,the absolute maximum error between the receiver frequency synthesizerand the received carrier can be as much as 2760 Hz(915E6*20E−6+925.7E6*10E−6). I.e., the resulting IF frequency is offsetfrom its nominal value by this amount. This represents 260 ppm of thenominal IF frequency. An ordinary frequency counter with a time baseaccuracy determined by the receiver crystal oscillator, i.e. +/−10 ppmcan detect this error and measure it with good accuracy. The accuracyshould be better than +/−110 Hz ({fraction (1/26)} of the maximumerror). Based on the measured frequency error, the relative frequencyoffset is calculated and the time correction factor for each transmitteris adjusted accordingly. For example, if the measured error is +1380 Hzthen the relative frequency error is approximately equal to +15 ppm. Ifthe nominal value of the time interval between two consecutivetransmissions is 100 seconds, then the required correction is +1.5 ms ifthe receiver uses high injection, i.e. the frequency of the synthesizerin the receiver is nominally equal to the received frequency plus the IFfrequency, and −1.5 ms if low injection is used.

[0062] In the advantageous embodiment, the time base correction factorstored for each transmitter is also used to adjust the center frequencyof the receiver and thus aid the reception of the transmitted packets,thus lowering the requirements for the length of the preamble includedin each packet for the purpose of carrier and data timing acquisition.This is accomplished by adjusting the receiver frequency momentarilyjust prior to the reception of the packet from a transmitter from whicha packet is due.

[0063] In operation, the receiver scans the alarm frequencies during thetime when it is not occupied with the scheduled reception from thetransmitters or checking the time registers. Also the receiver scans allthe available frequencies in addition to the alarm frequencies. Duringthe scan, the receiver uses RSSI signal to detect if there is an energytransmitted on a current frequency; if so, then the receiver measures apredetermined unique properties of the modulated carrier. If the energyis not present or the unique property is not valid, the receiver willquickly proceed to examine the next frequency. Otherwise the receiverwill stay on this frequency and try to decode the message. This way allthe alarm frequencies are examined several times per second ensuringthat the receiver can receive the alarm message with a minimum delay.Also, the scan of all available frequencies is fast; the synchronizationcan be regained faster and more reliably because the receiver will notwaste much of the time for an examination of very weak or spurioussignals.

[0064] In the advantageous embodiment, when a transmitter is powered up,for example after a battery replacement, it enters a power-up modeduring which a predetermined number of packets are transmitted on thealarm frequencies in a way similar to the transmission of alarm packets.In the power-up transmission sequence, each packet includes a numberthat indicates how many packets the transmitter has transmitted in thismode or how many packets the transmitter will transmit in this modebefore entering a normal mode of operation. This way, the receiver cansynchronize with the transmitter just after a single packet reception bycalculating when the first transmission will occur in the normal mode.

[0065] In the advantageous embodiment, the transmitter ID numbers foreach transmitter stored in the receiver ID memory registers 134 areacquired and stored by the receiver during a process of log-in. Each newtransmitter to be logged-in is placed in a relatively close proximity tothe receiver and then powered up. A very high level of the receivedsignal ensures that the new transmitter signal is not mistaken foranother transmitter. A successful log-in is confirmed by the receiverusing an audio or a visual indicator that can be included in thereceiver or in the system controller connected to the receiver viasystem interface 140. The receiver may reject the transmitters that cancause persistent collisions, i.e. if its ID number has the last 24digits identical to another transmitter already present in the system.

[0066] In the illustrative embodiment described here, references aremade to several elements such as generators, logic, registers, etc. Itis to be understood that various elements described here can be realizedin several different forms including software and hardware in theirvarious forms and combinations.

[0067] Although illustrative embodiments of the invention have beendescribed in detail herein with reference to the accompanying drawings,it is to be understood that the invention is not limited to thoseprecise embodiments, and that various changes and modifications can beeffected therein by one skilled in art without departing from the scopeand spirit of the invention as defined by the appended claims.

[0068] It is to be understood that the above-described embodiments aremerely illustrative of the invention and that many variations may bedevised by those skilled in the art without departing from the scope ofthe invention. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A wireless telemetry system comprising: aplurality of transmitters, each of which is for transmittingintermittently radio frequency transmissions independently of anyapparatus for receiving any of said transmissions, and a receiver forholding, simultaneously for each of said plurality of transmitters, timedata indicative of an expected time of at least one future transmission.2. The system of claim 1 wherein each of said transmitters is fortransmitting at varied frequency, and said receiver is for holding,simultaneously for each of said plurality of transmitters, frequencydata indicative of an expected frequency of at least one futuretransmission.
 3. The system of claim 2 wherein said receiver comprises afrequency selective circuit and, in operation, for each of saidplurality of transmitters, said receiver changes frequency of saidfrequency selective circuit to said expected frequency at such timerelative to said expected time to receive and demodulate said at leastone future transmission.
 4. The system of claim 1 wherein each of saidplurality of transmitters is for varying transmission frequencyaccording to a pattern that is different for each of said plurality oftransmitters.
 5. The system of claim 1 wherein each of said plurality oftransmitters is for varying transmission frequency and time betweentransmissions according to a frequency-time pattern that is differentfor each of said plurality of transmitters.
 6. A telemetry receivercomprising: logic for holding, simultaneously for each plurality ofradio frequency transmissions, data indicative of an expected time of atleast one future transmission, wherein each said plurality of radiofrequency transmissions is transmitted by a different one of a pluralityof transmitters, and wherein each of said plurality of transmitters isfor transmitting intermittently, independently of any receiver forreceiving any of said transmissions.
 7. The receiver of claim 6comprising logic for holding, simultaneously for each said plurality ofradio transmissions, data indicative of an expected transmissionfrequency of at least one future transmission, wherein each saidplurality of radio transmissions is at varied frequency.
 8. The receiverof claim 7 wherein said receiver comprises a frequency selective circuitand, in operation, for each of said plurality of transmissions, saidreceiver changes frequency of said frequency selective circuit to saidexpected frequency at such time relative to said expected time toreceive and demodulate said at least one future transmission.
 9. Thereceiver of claim 6, wherein said receiver, in operation, determinessaid expected time based on data included in said transmissions.
 10. Thereceiver of claim 6 wherein, for each said plurality of radiotransmissions, said receiver determines an expected frequency and anexpected time of said at least one future transmission based on dataindicative of a frequency-time pattern included in said transmissions.11. A telemetry transmitter comprising: a circuit for transmittingintermittently radio frequency transmissions, at varied transmissionfrequency, independently of any apparatus for receiving any of saidtransmissions, and logic for varying said transmission frequencyaccording to a frequency index varied in such a way that, upon eachtransmission, for at least one future transmission, said frequency indexis predictable based on frequency index of at least one pasttransmission.
 12. The transmitter of claim 11 wherein said transmissionsinclude data indicative of said frequency index variations.
 13. Thetransmitter of claim 1 wherein said transmitter comprises a number forselecting a sequence for varying said frequency index.
 14. Thetransmitter of claim 11 wherein said logic is for varying time betweentransmissions according to a frequency-time sequence and wherein saidsequence is determined based on a unique number included in saidtransmitter.
 15. The transmitter of claim 11 wherein said logic is forvarying time between transmissions according to a frequency-timesequence and said transmissions include data indicative of saidfrequency-time sequence.